According to a NeoGAF user jaaz, it appears that GameStop is desperate to sell Xbox Ones to customers

“I’ve seen the same or similar set-up at least two other GameStops. They had a few empty PS4 boxes stacked but nothing near as prominent. Inquiries on whether the PS4 was in stock were met with a negative. In the last GameStop, the employee actually said he had a PS4 but was using it as a Kinect stand for his Xbone because the PS4 has no games, especially sports games. I didn’t even bother with him.

Take it for what’s it worth, but there seems to be a clear push at GameStop to move Microsoft’s next-gen device. It also appears from the homemade signs that GameStop is still trying to clear the air from the Xbone reveal fiasco earlier this year.” – jaaz

Sony on the other hand doesn’t need to try extra hard to sell the console as it didn’t backtrack on any of its visions or promises for the console.